The invention relates to lamps for producing vacuum UV radiation, such as used in photoionization detection.
There is a need in photoionization detection to provide short wavelength (so called "vacuum") ultraviolet radiation, for detecting low-molecular-weight compounds (which have high ionization potentials). Typically, an argon-filled lamp is used, as it provides 11.6 and 11.8 eV emission lines. Such short-wavelength radiation will not pass through the glass walls of the lamp, and thus it is customary to use a lithium fluoride crystal as a transmission window. A persistent difficulty with such windows has been that they develop what are known as color centers, which absorb radiation in the vacuum UV range emitted by the lamp. This result in the windows rather rapidly becoming opaque to the vacuum UV radiation, greatly shortening the useful lives of the lamps (e.g., a 10.2 eV lamp that does not suffer from color center formation may have a useful life 100 times that of an 11.6 and 11.8 eV argon lamp with a lithium fluoride window).
This color center problem has received considerable attention in the published literature for more than two decades. E.g., Warnek, Peter, "LiF Color-Center Formation and uv Transmission Losses from Argon and Hydrogen Discharges", Journal of the Optical Society of America, August 1985, Volume 55, Number 8, pp. 921-925; Kittel, Charles, "Introduction to Solid State Physics", Wiley, 2 ed., pp. 491-497; Sampson, J. A. R., "Crystalline Materials", Tech. Vac. U.V., pp. 183-184 (published approximately 1965); McNesby, J., et al., "Vacuum Ultraviolet Techniques in Photochemistry", 1971, pp. 548-551; Warnek, Peter, "A Microwave Powered Hydrogen Lamp for Vacuum Ultraviolet Photochemical Research", Applied Optics, November 1962, Vol. 1, No. 6, pp. 721-726; Laufer, A. H. and McNesby, J. R., "Photolysis of Ethane at the Argon Resonance Lines 1067 and 1048A", J. Chem. Phys., p. 3329; Lane, Arthur L. and Kuppermann, Aron, "Argon Resonance Line Lamp for Vacuum Ultraviolet Photochemistry", pp. 126-127. Some of these publications offer suggestions for bleaching the color centers or otherwise restoring transmittancy: polishing (Sampson); exposure to a mercury lamp for a number of hours (McNesby et al.); bleaching by illumination with light absorbed in the F band (5 eV for lithium fluoride) (Kittel); heating at 350.degree. C. for about twelve hours (Laufer et al.). The present inventor several years ago considered adding mercury vapor to the argon, in an effort to retard color-center formation, but found that the mercury unduly degraded the 11.6 and 11.8 eV emission of argon.